1. Field of the Invention
The present invention relates to a labeled protein for obtaining the labeled protein in high purity and a method for obtaining the same. More specifically, the invention relates to a labeled antibody useful for diagnostic application and a purification method thereof.
2. Description of the Related Art
A labeled protein including a labeled antibody is used for diagnosis of lesions such as cancer. The labeled protein includes a protein binding to a molecule specifically present in a lesion part, and as a labeling substance, a signal-generating part which enables detection from the outside of the living organism or a chemical modifier which modifies physical and chemical properties of the protein. Since the labeled protein binds to the specific molecule and the labeling substance sends a signal, the detection of the signal enables the present/absence of the specific molecule and the amount and location information thereof to be obtained.
Upon using the labeled protein as a diagnosis agent or a contrast agent, an unlabeled protein mixed therewith causes competitive inhibition and thereby decreases the detection sensitivity. Therefore, a method for separating and purifying a labeled protein is desired.
However, it is difficult to separate a labeled protein and an unlabeled protein after labeling. In particular, when a low molecular compound such as fluorescent dye and water-soluble oligo (ethylene glycol) is used as a labeling reagent, the separation becomes further difficult. This is because labeling with a low molecular compound does not make a large difference between the labeled substances before and after labeling in the physical and chemical properties (a molecular size, hydrophilicity/hydrophobicity, an electric charge, etc.).
Size exclusion chromatography, ion-exchange chromatography and hydrophobic chromatography can be used to separate a labeled protein. However, in the case of labeling with a low molecular compound, it is not easy to separate a labeled protein and an unlabeled protein even with these chromatography methods, and detailed investigation of conditions is necessary. In stead of size exclusion chromatography, an antibody against a labeling reagent can be used for separation of a labeled protein. However, this method is not versatile because the antibody is not always available.
As a method for purifying a labeled protein, a method for separating a labeled protein from an unlabeled protein using hydrophobic chromatography (Japanese Patent Application Laid-Open No. H09-325142, separation purification method with a hydroxyapatite column (Japanese Patent Application Laid-Open No. H02-002937) are reported.